My personal finance blogs were started with $100, but you can start a blog with $20 if you buy hosting on a monthly basis. That’s 4 Starbucks coffees or 4 packs of cigarettes many paycheck to paycheck people do find a way to buy. After six months of HARD work, my first site started generating $2,000 a month, and today, those three sites generate over $5,000 a month, while all I have put in was hosting for $100-ish every year each, and a website redesign for under $1,000 after three years. Freelance writing and translation jobs are also a sizable part of my income that did not require any upfront investment. Investing $10 a month in index funds is also a realistic way for many to build yet another income stream.
It’s a (mostly) short term, higher risk, higher reward place to invest cash that has a low correlation with the stock market, but is far more passive than buying and managing properties, has more opportunity for diversification than private placements (minimums of 5-10K, rather than 100K), and most of the equity offerings (and all of the debt offerings) provide monthly or quarterly incomes. Unlike a REIT, you can choose exactly which projects you wish to invest in.

If you have a blog or other type of site, you can build affiliate links to different services on the website. Many people use Amazon as an affiliate partner. For example, if you are a beauty blogger writing about different products, you can set up Amazon affiliate links on your blog so that whenever someone buys the product you mention on Amazon, you receive a percentage of the sale. Amazon is not the only affiliate partner out there. Here is good, in-depth information on affiliate marketing. 
Injection means introduction of income into the flow. When households and firms borrow the savings, they constitute injections. Injections increase the flow of income. Injections can take the forms of investment, government spending and exports. As long as leakages are equal to injections, the circular flow of income continues indefinitely. Financial institutions or capital market play the role of intermediaries. This means that income individuals receive from businesses and the goods and services that are sold to them do not count as injections or leakages, as no new money is being introduced to the flow and no money is being taken out of the flow.
Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.